Porous carbon is used, for example, in electrodes for fuel cells, supercapacitors, electric accumulators (secondary batteries) and as adsorbents for liquids and gases, as a storage medium for gases, as a carrier material in chromatographic applications or catalytic processes, and as a material in mechanical engineering or medical technology.
Components of porous carbon foam have been known for a long time. This foam is obtained by heating organic starting materials to temperatures between 1000-1500° C. under inert gas. DE 69934256 T2, for instance, describes a method for producing open-pored, substantially graphitic, carbon foam by heating and coking pitch under non-oxidizing conditions under pressure.
Carbon foam is distinguished by an extremely low density of less than 0.1 g/cm3 and by a high temperature resistance of up to 4000° C. under inert gas. Typical minimal pore sizes are around 5 μm; for many applications, however, pores are needed in the nanometer range, as well as large specific surface areas that cannot be obtained with this material.
DE 29 46 688 A1 discloses methods for preparing porous carbon using a temporary preform of porous material (a so-called “template”). A precursor substance for carbon is deposited in the pores of the “template” consisting of inorganic template material which has a surface area of at least 1 m2/g. SiO2 gel, porous glass, aluminum oxide or other porous heat-resistant oxides are mentioned as suitable template materials for the template. The template material has a porosity of at least 40% and a mean pore size in the range of 3 nm to 2 μm.
Polymerizable organic materials, such as a mixture of phenol and hexamine or a phenol-formaldehyde resol, are recommended as the precursor substance for carbon. It is introduced as a liquid or as a gas into the pores of the template and polymerized. After polymerization and subsequent carbonization, the inorganic template material of the template is removed, e.g. by dissolution in NaOH or in hydrofluoric acid.
In this way, a particle- or flake-like carbon product is obtained which has a pore structure with macropores substantially reflecting the former template structure. This carbon structure can also contain micropores, which can be reduced or eliminated by a post-treatment such as coating with a pyrocarbon or by graphitization. The carbon product is suited for use in gas chromatography or as a catalyst carrier.
A so-called “hierarchical pore structure,” however, turns out to be advantageous for many applications. Large surfaces can be provided by pores in the nanometer range. To enhance the accessibility to said pores, these are ideally connected via a continuous macroporous transport system. A monolithic carbon product with such a hierarchical pore structure of macropores and mesopores is described in US 2005/0169829 A1. To prepare the hierarchical pore structure, a SiO2 template is produced by heating a dispersion of silica beads with diameters of 800 nm to 10 μm and of a polymerizable substance in a mold, so that a porous silica gel is obtained by polymerization, the gel being dried after removal of the excess liquid and completely polymerized.
The pores of the SiO2 template obtained thereby are subsequently impregnated with a precursor substance for carbon, which carbonizes the carbon precursor substance into carbon, and subsequently removes the SiO2 template by dissolution in HF or NaOH. The carbon product obtained thereby also shows a pore structure approximately corresponding to the material distribution of the template. Phenolic synthetic resin which is dissolved in tetrahydrofuran (THF) is here used as the precursor substance for carbon.
The common graphitizable carbon precursor substances for the infiltration are not soluble in high concentration and have a fraction of insoluble constituents. For instance, the solubility of pitches in THF is less than 10% by vol., so that after evaporation of the solvent more than 90% of the originally filled pore volume remains unfilled. The volume of the remaining coating of carbon precursor is further reduced by subsequent carbonization.
Conversely, carbon precursors in the form of carbohydrates—such as sugars—exhibit high solubility in solvents, but the sugar remaining after evaporation of the solvent loses up to 75% of its original mass in the carbonizing process, so that a large pore volume also remains unfilled in this instance. Therefore, these carbon precursors will normally yield only small thicknesses of the deposited carbon layer. In order to achieve technically reasonable wall thicknesses of the porous carbon structure, several of these infiltration and carbonization processes have to be carried out one after the other as a rule. These multiple processes, however, increase the manufacturing costs and can cause inhomogeneities, for instance as a result of the gradual clogging of infiltration channels.
To mitigate this problem, WO 201211966 A1 suggests a modification of the production of porous carbon using porous template material with a hierarchical pore structure. Powders produced in advance, both from the porous template material and from the precursor substance, are provided. These powders are mixed together homogeneously and the homogeneous powder mixture is heated to such an extent that the particles of the precursor substance melt; the precursor substance melt can penetrate into the pores of the template. A solvent for the carbon precursor substance may also be omitted. A uniform distribution and occupation over the entire pore volume of the template material to be infiltrated is attained, so that even with only a one-time infiltration one already achieves a high filling degree of the pore volume.
The carbonization of the precursor is carried out at the same time as or subsequent to the infiltration of the pores of the template particles. The concurrent shrinkage of the precursor substance is due to the decomposition and evaporation processes during carbonization. The inorganic template material serves only as a mechanically and thermally stable framework for depositing and carbonizing the carbon precursor substance.
After removal, for instance by chemical dissolution, the resulting carbon product is substantially free of template material. It shows a finely rugged surface which is crisscrossed in the form of channels by a multitude of coherent pores and voids of different sizes.
The technical article “Nanocasting—A Versatile Strategy for Creating Nanostructured Porous Materials” by An-Hui Lu, Ferdi Schüth, published in Adv. Mater. (18), 1793-1805 (2006), describes a template method in which mesoporous SiO2 is infiltrated with a precursor substance for carbon and carbonized, and the SiO2 framework is dissolved. This yields mesoporous carbon.